A great application that doesn't always get the lime-light is Terragen. It's slow in development but it's new features that will be coming in Terragen 2 are really impressive.
Terragen is capable of photorealistic results for professional landscape visualisation, special effects, art and recreation.
What is great about Terragen is the amazing ability to create spectacular photorealistic scenery and then you can export the terrain to other programs such as Blender for use in other projects. The new version has advanced shaders and terrain generation capabilities.
Terragen 2 is not just for stills either, you can use it for animations. The creators over at Planetside Software have created an incredible video that demonstrates the power of the animation engine. You can check out the Terragen 2 Mars Animation here (remember to right click Save As).
To get started with Terragen read this guide on Surface Mapping.